With increasingly strict quality control demanded of steel products, the buildup of non-metallic inclusions, such as alumina, on the wall surface of an inner hole of a submerged or immersion nozzle has a serious disadvantage for steel production. Therefore, a good deal of effort has been made to reduce the buildup of non-metallic inclusions, such as alumina, on the wall surface of an inner hole of an immersion nozzle for continuous casting (hereinafter referred to as “immersion nozzle” or “nozzle”).
As one of measures against the alumina buildup, there has been known a technique in which argon gas is injected from the inner surface of a nozzle into molten steel to prevent the alumina buildup in physical manner. In this technique, if the argon gas is injected excessively, bubbles of the injected argon gas will be incorporated into molten steel to form pinholes in slabs. Thus, due to the restriction in the allowable injection amount of argon gas, this method cannot be fully employed as a sufficient measure to prevent alumina buildup.
There has also been known a technique intended to provide an anti-alumina-buildup function to refractories themselves. For example, Japanese Patent Laid-Open Publication No. 57-71860 discloses a casting nozzle using refractories including a combination of graphite and either one of sintered calcia, fused calcia and another ceramic material containing a CaO component. This invention is intended to induce a reaction between CaO contained in the refractories and alumina attached on the wall surface of a nozzle inner hole so as to form a low-melting-point or fusible substance to prevent the alumina buildup.
Such CaO-containing refractories exhibit a certain level of anti-alumina-buildup effect. In particular, refractories using a clinker containing CaO as a mineral phase, such as calcia-magnesia clinkers including calcia clinker and dolomite clinker, have an excellent effect.
For this reason, immersion nozzles have been increasingly produced by integrally molding a compound using a clinker containing CaO as a mineral phase, which is applied to only an inner hole portion of an immersion nozzle or an nozzle body including the inner hole portion, and a zirconia-graphite compound applied to a powder-line portion of the nozzle, and then burning the molded piece in a reduction atmosphere.
This production method involves a problem concerning cracks to be generated at the boundary between the different compositions during the burning process. This would be because phenol resin, which is added to the CaO-containing refractories at about several % to less than 20% as a binder or for forming a carbon bond in the refractory matrix, decomposes during heating to release water therefrom, and the resulting hydration reaction between the water and the CaO contained as a mineral phase in the clinker causes volume expansion different from the expanding behaviors in other portions during heating.
Japanese Patent Laid-Open Publication No. 2000-514394 proposes one measure against this problem, in which a CaO-containing powder is added into a zirconia-graphite compound for use in a powder-line portion so as to equalize expanding behavior in each portion to prevent the occurrence of cracks. However, the CaO-containing powder added to the zirconia-graphite compound causes deterioration in corrosion resistance, resulting in undesirably deteriorated durability of a powder-line portion to be formed of the compound.